It is known in the art to produce thermoplastic resin particles imbibed with a blowing agent for expansion at a later time. In the case of polyphenylene ether foam products, generally a blowing agent such as pentane is impregnated under pressure in polyphenylene ether. The impregnated particles are then heated, usually in steam and the particles foam. The foamed particles can then be placed in a mold and heated again to the point where the particles fuse. The result is a quite rigid low density product which has many uses from packaging to insulating board.
There are also many disclosures in the art for imbibing polystyrene resin particles. Early processes are summarized in Frisch & Saunders, "Plastic Foams", Marcel Deleker, Inc., N.Y., 1973, Part II, pp. 525-544, such as diffusion of blowing agents into polystyrene, polymerization of styrene solutions of volatile hydrocarbons, suspension polymerization systems, deposition of expandable polystyrene from solution, quenched-pellet processes and water-in-monomer polymerizations.
There have been a variety of attempts to produce expandable pellets by incorporating the blowing agent in a polymer melt in an extruder and cutting the extrudate into pellets. Collins, in U.S. Pat. No. 3,250,834 extrudes strands of polystyrene containing pentane into grooves onto rotating rolls maintained at a temperature below the expanding temperature of the extruded material. Other disclosures employ underwater pelletizers, see e.g. Biglione et al, U.S. Pat. No. 4,606,873; Gwinn, U.S. Pat. No. 4,385,016; Muirhead et al, United Kingdom Pat. No. 1,062,307; Suh, U.S. Pat. No. 4,386,165; and Allen et al in commonly assigned U.S. patent application Ser. No, 093,317, filed Sept. 4, 1981. Hambrecht et al in German Patent DE 3,220,856 discloses melting a polyphenylene ether/polystyrene blend and mixing with a blowing agent at a pressure of 25 to 250 bars, but does not describe making expandable pellets.
It is also known to incorporate a blowing agent into the thermoplastic resin during polymerization. Kajimura et al, U.S. Pat. No. 4,442,232 teaches the addition of the blowing agent to the reaction system in the production of expandable styrene-maleic anhydride copolymers. See also Hahn et al, U.S. Pat. No. 4,459,373.
At present the most common method is to impregnate the thermoplastic particles with blowing agent in a suspension. Kajimura et al, U.S. Pat. No. 4,303,756 describes impregnating thermoplastic resin beads with blowing agent in an aqueous suspension under pressure. Krutchen et al, in U.S. Pat. No. 4,532,263 discloses imbibing polyetherimide resins in an excess of a solvent selected from the group consisting of methylene chloride, chloroform and 1,1,2-trichloroethane at temperatures up to about 100.degree. F. DiGiulio, U.S. Pat. No. 4,661,386 discloses carrying out the impregnation of pentane in polystyrene in a slurry of water at 90.degree. C. stabilized by finely divided calcium phosphate and an anionic surfactant Allen et al, in commonly assigned U.S. Pat. No. 4,782,098 discloses suspending the thermoplastic resin beads in water containing a suspending agent in an autoclave, heating the suspension and introducing the blowing agent under pressure to produce the expandable thermoplastic beads.
None of these imbibing processes however teach the use of a low intrinsic viscosity (I.V.) polyphenylene ether as the thermoplastic resin and the advantages associated therewith. All intrinsic viscosity values employed in this specification are measured in chloroform at 25.degree. C. unless otherwise indicated. By low intrinsic viscosity is meant, polyphenylene ethers having an I.V. of less than 0.45 dl/g, preferably less than about 0.40 dl/g, and more preferably less than about 0.30 dl/g. The most preferred polyphenylene ether resin is one having an I.V. of from about 0.10 dl/g to about 0.30 dl/g. Surprisingly, when lower I.V. polyphenylene ethers are employed in producing expandable microparticles, there results a significant decrease in molding cycle times, molding pressures and in the amount of residual blowing agent in the final molded part.
Low intrinsic viscosity polyphenylene ether foamable resins are disclosed in Allen et al commonly assigned U.S. Pat. No. 4,727,093 which describes a process for making low density polyphenylene ether/polystyrene foams with polyphenylene ethers having an intrinsic viscosity of about 0.1 to 0.7 dl/g, but does not teach producing expandable beads or the advantages associated therewith. Allen et al in commonly assigned U.S. patent application Ser. No. 093,317, filed Sept. 4, 1987, broadly describes an underwater pelletizing process for producing expandable beads employing polyphenylene ether resins having an I.V. of about 0.1 to about 0.7 dl/g, but does not teach the molding advantages associated with employing polyphenylene ether resins having an I.V. of less than about 0.45 dl/g.